Aluminum-lithium scrap recovery

ABSTRACT

A process is provided for melting and reclaiming aluminum and lithium from aluminum-lithium scrap including heating a molten salt in a heat bay, mixing the heated salt with aluminum-lithium scrap in a charge bay to form a molten mixture, separating aluminum from impurities in the charge bay by coalescing aluminum to form a molten aluminum metal pad, chlorinating a portion of the charge mixture from the charge bay to form a chlorinated salt mixture and metal chlorides, removing metal chlorides from the salt mixture, removing lithium from the salt mixture, and feeding the salt mixture back to the heat bay. In one aspect, the chlorinating step of the present invention includes introducing carbon monoxide or, preferably, solid carbon to control oxide concentration. The process, in other aspects, further includes adding fluorides and recovering lithium and removing metal chlorides from the salt to maintain a preferred salt composition. The lithium and the metal chlorides can be removed by withdrawing volatile metal chloride gases and electrolytically reducing metal chlorides in the molten salt.

BACKGROUND OF THE INVENTION

This invention relates to a process for recovering aluminum-lithium fromaluminum-lithium alloy containing impurities such as fromaluminum-lithium scrap or aluminum-lithium skim or dross.

Aluminum metal is an engineering material which can be readily recycled.Aluminum recycling involves recovery or reclamation of aluminum metalfrom aluminum scrap containing impurities, e.g., such as aluminum scrapor aluminum skim or dross from processes wherein molten aluminum metalcomes in contact with oxygen in the air. Typically, the impurities inthe aluminum exist as oxides and include aluminum oxide on the surfaceof the aluminum and other oxides such as surface magnesium oxidederiving from alloying elements. Aluminum skim often contains nitridessuch as AlN and carbides such as AlC. Other extraneous materials arepresent in aluminum-lithium scrap. An example of such extraneousmaterials in aluminum-lithium scrap would be surface oxides containinglithium aluminate and oxides of aluminum, lithium, and magnesium.

Molten salt reclamation processes involve lifting non-metallic materialsfrom aluminum or aluminum alloy during melting and preferentiallywetting the non-metallic materials comprising the impurities in theincoming aluminum scrap. The molten salt preferentially wets theimpurities which separate from the aluminum as the aluminum coalescesinto metal droplets. The aluminum metal droplets sink to the bottom of asalt-containing process vessel and form into a continuous molten metalpad of aluminum.

The preferential wetting of the oxides and the resulting separation ofaluminum from such contaminant oxides is a function of salt compositionand the amount of solid particulate oxides and nitrides present in thesalt. As scrap is fed to the salt-containing process vessel, the amountof oxides present in the salt increases with each increment of scrapadded to the process. This increasing oxide content poses a seriousproblem in salt-based reclamation processes since the presence of oxidesat levels higher than 5 to 30 wt %, as a function of particle sizedistribution, causes significant reductions in the recovery of aluminumfrom the process. In effect, the high oxide levels poison the processfor reclaiming aluminum metal. This aspect of the conventional processeshas been a significant drawback to the efficiency and economics ofsalt-based recovery systems.

It is an object of the present invention to provide a process forreclaiming aluminum metal from aluminum-lithium scrap containingimpurities.

It is a further object of the present invention to provide a process forreclaiming aluminum metal from aluminum-lithium scrap containingimpurities at a higher yield than in present processes.

It is yet another object of the present invention to provide a processfor melting aluminum-lithium alloy while preventing contamination of themelt to an extent that is not now practicable.

These and other objects of the present invention will become apparentfrom the description of the invention as follows.

SUMMARY OF THE INVENTION

The process of the present invention for continuous salt-based meltingand recovery of aluminum-lithium includes heating a molten salt in aheat bay, mixing the heated salt with aluminum-lithium scrap in a chargebay to form a molten charge mixture, separating aluminum metal fromimpurities by coalescing to form a molten metal pad of aluminum and asalt sludge containing lithium and separated impurities, chlorinating aportion of the salt sludge to form a chlorinated salt mixture containinglithium and metal chlorides, removing metal chlorides from thechlorinated salt mixture, removing lithium, and feeding the remainingchlorinated salt mixture back to the heat bay. The process chlorinatingstep includes using carbon or carbon monoxide to control oxideconcentration. In one aspect, carbon is the preferred agent to controloxide concentration. The step for removing metal chlorides includeswithdrawing metal chloride gas and further includes reducing moltenmetal chlorides and lithium electrolytically and removing reduced metalso produced.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a schematic diagram depicting the overall process ofthe present invention.

DETAILED DESCRIPTION

The process of the present invention represents a continuous deep saltmetal reclamation technique designed and developed primarily for meltinghigh surface area, dirty aluminum-lithium scrap. Aircraft plate andsheet are examples of such aluminum-lithium scrap, and the process ofthe present invention is suitably tailored to an increased volume ofsuch aircraft plate and sheet expected to be available for recycling inthe future. However, the process of the present invention is not limitedto aircraft plate or sheet but can be applied to most anyaluminum-lithium containing oxide impurities including aluminum-lithiumskim or dross formed in melting processes wherein moltenaluminum-lithium alloy comes into contact with oxygen in the air.

The continuous metal scrap reclamation process of the present inventioninvolves a molten salt maintained continuously in the molten state andproviding a chemical composition and oxide composition such that metalrecovery is achieved at improved levels continuously.

The process of the present invention operates to control and maintainchemical composition concentration continuously at a level consistentwith the maximum recovery of aluminum and lithium metal through reactionof the oxides with chlorine and carbon or carbon monoxide to formchlorides of two general types: volatile and non-volatile. The volatilechlorides, e.g., such as silicon tetrachloride, are removed from thecontinuous salt-based melting process as part of the reaction sequencein the carbon monoxide/dioxide vapor waste stream. Small amounts of allchlorides present, including aluminum chloride, are removed in theoffgas stream, but most remains in the molten salt.

Non-volatile chlorides, e.g., such as aluminum chloride, lithiumchloride, and magnesium chloride, are removed in a process step byelectrolysis. The process provides a continuous melting operation inwhich recovery of molten aluminum and lithium is maximized bycontrolling the integrity of the salt-based melting medium continuously.

Referring now to the FIGURE, a continuous scrap remelt process involvesa superheated molten salt melt medium circulated throughout thecontinuous process of the present invention beginning, for purposes ofillustration, in heat bay 1. Salt is heated in heat bay 1 to atemperature maintained in the range of about 1400°-1450° F. The salt ispumped from heat bay 1 by pump 2 to charge bay 3 where the salt is mixedwith aluminum-lithium scrap through an appropriate agitation in a swirlmotion. In one aspect, scrap enters charge bay 3 continuously via afume-controlling lock chamber (not shown). As the heated salt mixes withthe aluminum-lithium scrap, a coalesced molten metal collects in a lowerportion of a collection bay depicted here as collection bay 4 where anoptional coalescer is located to insure substantially metal-freesalt/oxide passage to chlorination bay 6. Chlorine and carbon or carbonmonoxide, alternatively phosgene, is introduced to the salt stream inchlorination bay 6 to convert the required amount of oxides intochlorides to maintain a steady-state oxide concentration. In thechlorination bay 6, several gas species are provided such that carbondioxide is the main reaction by-product as well as titaniumtetrachloride and silicon tetrachloride if titanium or silicon ispresent as scrap contaminant which, together with excess chlorine andcarbon monoxide, are withdrawn from chlorination bay 6 and passedthrough a scrubber (not shown). The molten salt, now enriched withaluminum chloride and magnesium chloride, passes through an optionalfilter (not shown) to electrolysis bay 9 in which the required amount ofreduction of lithium, aluminum, and magnesium is controlled to removethe lithium and maintain a preferred steady-state salt concentrationcondition. Lithium and a relatively small amount of aluminum andmagnesium metal formed in this step can be tapped periodically. Cleanedsalt passes from electrolysis bay 9 and is fed back into heat bay 1 tobe superheated again and maintained at process temperatures of about1400°-1450° F.

The means for heating the salt melt medium in heat bay 1 preferably isAC resistance heating. Heating provided by AC resistance minimizes fumegeneration and can be carried out under an inert atmosphere.

The process of the present invention takes place within two or moreinterconnected chambers, as depicted in the FIGURE. Molten salt flowscontinuously through the chambers, e.g., as shown in the FIGURE incounterclockwise flow. The flow of salt in the process is induced by amolten salt pump as depicted by pump 2 in the FIGURE. Multipleelectrodes can be used in the heat bay to transfer electrical energy tothe salt.

Salt leaving heat bay 1 enters charge bay 3, the next major chamber ofthe process. In the charge bay, as scrap is added continuously to aswirling or agitating salt, movement of the salt aids in the separationof oxides from the aluminum during melting. Flow rates of the salt intothe charge bay 3 are controlled in the process of the present inventionto be directly related to the scrap charge flow rate and incoming salttemperature. In this way, the process of the present invention providesa superheated salt (above about 1350° F.) for establishing the heatnecessary to melt the aluminum-lithium scrap.

The molten salt of the present invention comprises a salt containingabout 75-98% chlorides of sodium, potassium, magnesium, aluminum,calcium, and lithium. Other chlorides such as barium chloride or othersare not preferred for reasons that they are either more expensive ormore hazardous to handle. For aluminum-lithium alloys, the preferredsalt typically contains only chlorides of lithium and potassium. Thesalt further contains up to about 25%, preferably about 2-20%, and morepreferably about 5-15% by weight fluorides of one or more of sodium,potassium, magnesium, calcium, aluminum, and lithium. The composition ofthe salt is selected to control performance of the process in the threeprocess steps of charge mixing/coalescence, chlorination, and reductionat the lowest practical temperature. Salt melting temperature range inthe process of the present invention preferably is as low as possiblebut must be higher than the scrap melting point. Fluoride presence inthe salt is important for the reason that fluoride increases metalcoalescence in the charge bay and further for the reason that kineticsof the chlorination reaction are enhanced. At high fluorideconcentrations, higher temperatures would be required. However, highfluoride concentration is preferred because the chlorination reactionrate is higher, thus decreasing the size of the chlorination unit. Theincreased reaction rate is believed to be attributable to an increasedsolubility of oxide.

In one aspect, a salt containing 60-90% LiCl, 0-25% KCl, and 5-15% LiFis preferred. Outside this range of salt composition, coalescence and/orchlorination reaction rates decrease at the preferred operatingtemperature.

In charge bay 3, oxide films such as Al₂ O₃ and MgO from the scrap andtramp oxides present in the scrap stream such as TiO₂ and SiO₂ arewetted by the salt preferentially to form a salt sludge. The aluminummetal droplets sink to the bottom of charge bay 3 and coalesce into acontinuous body or pad of molten metal, which is tapped from the chambercontinuously or semi-continuously by a number of conventional tappingmethods.

Salt flows from the charge bay into the chlorination chamber wheresufficient oxides are reacted with chlorine and carbon or carbonmonoxide to maintain the concentration of oxides at the desired level.The level of oxides should be maintained at less than about 10 wt %. Allthe oxides are chlorinated by a reaction such as:

    MO.sub.x (s)+Cl.sub.2 (v)+CO(v)→MCl(l,v)+CO.sub.2 (v)

where M can be Al, Mg, Ti, Si, or other metals.

It has been found that solid carbon used in place of carbon monoxide gasvapor in the process of the present invention and mixed with chlorineprovides an unexpectedly advantageous result. In this aspect, excesscarbon is added and remains continuously with the salt. It would havebeen expected that carbon monoxide would react at a higher reaction raterather than solid carbon. However, it has been found empirically thatcarbon in the process of the present invention provides a reaction ratetwo times faster than when carbon monoxide is substituted in the sameprocess.

It has been found further that the chlorination step of the process ofthe present invention as depicted in the above equation will not go toany significant rate without the addition of carbon or carbon monoxide.

Metal chlorides such as silicon tetrachloride leave the chlorinationchamber in an offgas stream which also contains carbon dioxide,unreacted carbon monoxide, and chlorine gas. Other metal chlorides, suchas magnesium chloride, have sufficiently low vapor pressures attemperatures of about 1400° F. and do not leave with waste gas butrather build up in the salt. Aluminum chloride builds up in the saltbecause it readily forms low vapor pressure complexes with mostchlorides, e.g., such as NaAlCl₄ or KAlCl₄. Aluminum chloride wouldleave with the waste gases if the complexes were not formed since theatmospheric sublimation point of aluminum chloride is 365° F.

The final chamber through which the salt passes is called the reductionbay wherein the buildup of non-volatile metal chlorides such as MgCl₂,LiCl, and AlCl₃ is controlled. In the reduction bay, a low voltage DCcurrent passes through the salt to form electrolytic products ofchlorine gas and reduced metals. The chlorine gas so formed can berecycled to the chlorination reactor of the present process. The metalsare formed at the bottom of the chamber (the formed metal being thecathode) and can be removed periodically as moltenlithium-aluminum-magnesium alloys. Incoming scrap containing significantlithium, e.g., about 2 wt %, reacts with the AlCl₃ and MgCl₂ in thecharge bay, thereby forming aluminum, magnesium, and lithium chloride.Operative chemical reactions are:

    2AlCl.sub.3 +3Mg→3MgCl.sub.2 +2Al

    AlCl.sub.3 +3Li→3LiCl+Al

    MgCl.sub.2 +2Li→2LiCl+Mg

In this case, the only metal recovered in the electrolytic cell to anyextent is lithium, since AlCl₃ and MgCl₂ in the continuous process ofthe present invention will not build up and LiCl will increase inconcentration in the salt at a higher rate, if electrolysis is notcontinuous. If the electrolysis/reduction is carried out continuously,an Al-Mg-Li alloy is formed. However, a more pure Li alloy (low Al) andMg is formed as a by-product by allowing the AlCl₃ and MgCl₂ to reactthrough many charge/chlorination cycles before operating a batchelectrolysis reduction cell.

The preferred method for heating the molten salt-based melt medium ofthe process of the present invention is electrical resistance ACheating. However, electric radiation or other methods are suitable assubstitutes.

The process of the present invention is controlled to minimize anycontact with moist air. Otherwise, hydrolysis will occur, leading tochlorine losses. Additionally, fumes from the process would beenvironmentally undesirable. For these reasons, atmospheric contact ismaintained at a minimum.

The process of the present invention provides a chlorination step and anelectrolysis step operated continuously. However, suchchlorination/electrolysis steps can be performed in a batch orsemi-continuous reactor.

In one aspect, the chlorination step can be performed under a highpressure, e.g., above about 50-60 psig. Such a pressure increasesreaction rates significantly. However, the process of the presentinvention is not limited to such higher pressures for practical reasons.

While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass allembodiments which fall within the spirit of the invention.

What is claimed is:
 1. A continuous process for melting and purifyingaluminum-lithium scrap containing impurities comprising:(a) heating amolten salt in a heat bay; (b) mixing the heated salt with saidaluminum-lithium scrap in a charge bay to form a molten charge mixture;(c) separating aluminum metal from said impurities in said charge bay bycoalescing aluminum to form a molten aluminum metal pad; (d)chlorinating a portion of said charge mixture from said charge bay in asufficient amount to control oxide concentration in the charge mixtureto less than 10 wt % and to form a chlorinated salt mixture containingmetal chlorides; (e) separating and removing metal chlorides from saidsalt mixture; (f) removing lithium from said salt mixture; and (g)continuously feeding said salt mixture back to said heat bay.
 2. Aprocess as set forth in claim 1 wherein said molten salt comprises about60-90% wt % LiCl, 0-25 wt % KCl, and 5-15 wt % LiF.
 3. A process as setforth in claim 2 wherein said chlorinating comprises adding carbon orcarbon monoxide to control oxide concentration in the salt mixture.
 4. Aprocess as set forth in claim 3 wherein chlorinating comprises addingcarbon rather than carbon monoxide to control oxide concentration.
 5. Aprocess as set forth in claim 4 wherein said chlorinating forms volatilemetal chlorides as gases and non-volatile metal chlorides as moltensalts.
 6. A process as set forth in claim 5 wherein said removing stepfurther comprises reducing said non-volatile metal chlorides by anelectrolytic reduction.
 7. A process as set forth in claim 6 whereinsaid removing comprises scrubbing metal chloride gases to separate themetal chloride from carbonaceous vapor.
 8. A process as set forth inclaim 7 further comprising condensing silicon tetrachloride or titaniumtetrachloride prior to said scrubbing.
 9. A process as set forth inclaim 8 wherein said heat bay employs AC resistance heating and saidelectrolytic reduction employs DC electrodes.
 10. A process as set forthin claim 9 further comprising removing reduced metals formed by theelectrolytic reduction of the non-volatile metal chlorides.
 11. Aprocess as set forth in claim 10 wherein said impurities comprise theoxides of aluminum, magnesium, silicon, or titanium.
 12. A process asset forth in claim 11 wherein said aluminum-lithium scrap comprisesrecycled aircraft plate or sheet.
 13. A process for melting andrecovering aluminum and lithium from aluminum-lithium scrapcomprising:(a) heating a molten salt containing more than about 75 wt %chloride salts and up to about 25 wt % fluoride salts in a heat bay: (b)mixing said heated salt with aluminum scrap in a charge bay to form amolten charge mixture; (c) separating said aluminum from impurities inthe molten charge mixture by coalescing aluminum metal and forming amolten aluminum metal pad: (d) chlorinating a portion of said chargemixture from said charge bay by introducing chlorine gas and solidcarbon to form a chlorinated salt mixture and metal chlorides from theimpurities in said scrap in a sufficient amount to control oxideconcentration in the charge mixture; (e) removing metal chlorides tomaintain a specified concentration of metal chlorides in said saltmixture; (f) removing lithium from said salt mixture; and (g) recyclingsaid salt mixture back to said heat bay.
 14. A process as set forth inclaim 13 wherein said specified concentration of fluorides is about 2 toabout 20 wt %.
 15. A process as set forth in claim 14 wherein saidspecified concentration of fluorides is about 5 to about 15 wt %.
 16. Aprocess as set forth in claim 15 wherein said removing comprisesreducing metal chlorides electrolytically and removing the reducedmetals so produced.
 17. A process as set forth in claim 16 wherein saidaluminum-lithium scrap comprises aircraft plate or sheet.
 18. Acontinuous process for melting and recovering aluminum and lithium fromaluminum-lithium scrap comprising:(a) heating a molten salt of chloridesand fluorides in a heat bay to a temperature in the range of about1400°-1450° F.; (b) mixing the heated salt with aluminum-lithium scrapin a charge bay to form a charge mixture; (c) separating aluminum fromimpurities in said charge mixture by coalescing aluminum metal to form amolten aluminum metal pad; (d) chlorinating a portion of said chargemixture in a sufficient amount to control oxide concentration in thesludge to less than 10 wt % in the presence of solid carbon to form achlorinated salt mixture and metal chlorides; (e) removing metalchlorides from said salt mixture to maintain a specified chlorideconcentration of about 75-98 wt %; (f) removing lithium; and (g) feedingsaid salt mixture to said heat bay.
 19. A process as set forth in claim18 wherein said specified chloride concentration comprises 85-95wt % onan oxide free basis.
 20. A process as set forth in claim 19 wherein saidsalt comprises about 60-90 wt % LiCl, 0-25 wt % KCl, and 5-15 wt % LiFand said removing lithium step further comprises reducing lithiumchlorides in an electrolytic reduction to form reduced lithium metal andremoving the reduced lithium metal from the salt mixture prior to thestep of feeding the salt mixture back to the heat bay.